The present invention relates to a digital controlled power supply for turning a switching element on/off in response to a Pulse Width Modulation (PWM) signal to obtain an output voltage required for a load from an input voltage inputted thereto.
Due to the recent development of the semiconductor fine machining technology, there have appeared highly functional microprocessors having higher performance. Also, due to reduction in the operation voltage and the increase in the clock frequency, the current consumed in the processor and the change rate per unitary time thereof become larger. The tendency toward a lower voltage and a larger current causes a serious problem when supplying power to the processor. To cope with the problem, a Point-of-Load (POL) converter is used in ordinary cases.
The POL converter is a very small power source and is disposed at a position in the vicinity of the load to minimize line impedance between the power supply and the load of the microprocessor. To design and to install the POL converter, it is required to sufficiently give consideration to changes in various parameters caused by, for example, variations in the environmental temperature and the load. As such parameter variation, there has been well known a direct-current (DC) bias characteristic of a choke coil. This is a phenomenon in which when a direct current flows through the choke coil, inductance thereof lowers. Due to reduction in the inductance, the current further flows and an output ripple voltage becomes greater, leading to deterioration in stableness of the circuit operation and reduction in the circuit performance. It is likely in the worst situation that constituent components such as an output capacitor or condenser and IC circuits are destroyed.
To overcome such problems, there has been known a method of using a choke coil including a core made of a material having a high saturation magnetic flux density. Furthermore, there has been known a technique to improve efficiency of the power supply by reversely employing the DC bias characteristic in addition the coil superior in the DC bias characteristic (reference is to be made to, e.g., JP-A-6-319260). According to the technique, while the switching frequency is set to a low value in a low-load operation, there is utilized a coil having a particular DC bias characteristic of which inductance becomes larger in the low-load operation to suppress the output ripple voltage to a low value. This resultantly much more improves the efficiency of the power supply.